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Radar Cross Section of Moving Objects

H. Gholizade∗

Engineering Science Department, University of Tehran, Tehran, Iran.

Abstract

I investigate the effects of movement on radar cross section calculations. The results show that

relativistic effects (the constant velocity case) can change the RCS of moving targets by changing

the incident plane wave field vectors. As in the Doppler effect, the changes in the fields are

proportional to vc. For accelerated objects, using the Newtonian equations of motion yields an

effective electric field (or effective current density) on the object due to the finite mass of the

conducting electrons. The results indicate that the magnetic moment of an accelerated object is

different from that of an un-accelerated object, and this difference can change the RCS of the

object. Results for moving sphere and non-uniformly rotating sphere are given and compared with

static (v=0) case.

∗ gholizade@ut.ac.ir

1

I. INTRODUCTION

After Einstein’s theory of relativity, we know that some quantities, such as four-vectors,

have a norm which is invariant under Lorentz transformations. Maxwell’s equations have

a relativistic nature and we can calculate the relativistic effects on their solutions. The

most well known effect is the Doppler effect. The Doppler effect can be described by the

phase invariance of the wave equation. The relativistic Doppler effect has new consequences,

indescribable in the Newtonian point of view. For example, the transverse Doppler effect,

i.e., when the source velocity and the propagation direction are orthogonal. In the non-

relativistic Doppler effect, the frequency change is proportional to the cosine of the relative

angle between the directions of velocity and propagation. But in the relativistic case, we

know that if cos(θ) = 0 then we have a frequency shift [1]. This effect is different from the

rotational Doppler effect, because a rotating frame is not an inertial frame, so we can not

use the special theory of relativity to describe its physics. In the case of rotating frames, we

can use ”Franklin” transformation [2]. In relativity the linear velocity of rotating frame is

v(r) = c tanh( rωc) against v(r) = rω. Several authors discuss about relativistic corrections of

EM fields for both constant velocity and rotating frames [3–14]. At first sight, it seems that

relativistic effects can not be detected, but we know that relativistic effects in engineering

can be considered [15]. In this paper I focus on both relativistic effects (constant velocity)

and non-relativistic effects (accelerated objects) on the scattering process that can change

the differential scattering cross section of the scatterer. (The differential scattering cross

section is proportional to the radar cross section in engineering [16] by the factor 4π.) I

ignore the changes in frequency due to target movement (the Doppler effect), changes in

the incident angles, charge and current densities, target and source relative distances, and

only investigate the changes in the fields and their effects on the scattering cross section.

In this paper I focus on multi- static RCS (differential cross section), and express it as a

perturbative series (σ = σ0(1 + δ)) up to vcorder. I also solve equations for simple case

(moving sphere) and compar the results with static case (v = 0). In the relativistic case,

I use standard Lorentz transformations to find the observed fields in the different frames.

This process can be found in most textbooks [1, 16]. In the case of accelerated motion, I

use the effect of the conducting electrons’ finite mass (the Stewart–Tolman effect) [17] and

its effects on the current distribution. The result of Stewart–Tolman effect can be collected

2

as an effected magnetic moment [17]. The scattering cross section in Rayleigh region was

calculated for non- uniformly rotating sphere. First, I give a short review of the special

theory of relativity in Section II. Section III discusses the RCS of moving objects. Section

IV investigates the effects of target acceleration on the RCS, and the results for a simple

model (Rayleigh scattering for a conducting sphere) are given.

II. RELATIVITY IN ELECTRODYNAMICS

Let K and K ′ be inertial frames and suppose that K ′ moves with constant velocity v

relative to K. The observer O (i.e., in frame K) measures the electromagnetic fields H, E,

and observer O′ (in frame K ′) also measures the same fields as H′ and E′. To obtain the

relation between the fields in the two frames, we can use the field strength tensor [1]:

F µν = ∂µAν − ∂νAµ. (1)

Here, Aµ (µ = 0, 1, 2, 3) is the potential four-vector. The transformed tensor in the frame

K ′ can be written as

F ′µν =∂x′µ

∂xα∂x′ν

∂xβF αβ. (2)

Using the Lorentz transformation for x′µ we find the following relation between the fields in

the two frames [1, 16]:

E′ = γ(E+ β ×B)−γ2

γ + 1β(β.E), (3)

B′ = γ(B− β × E)−γ2

γ + 1β(β.B).

If we keep only terms of order vc, then [1, 16] (β = v

c, γ−1 =

√1− β2):

E′ = E+ β ×B, (4)

B′ = B− β × E.

Therefore, as the moving observer sees a different frequency, they measure different fields

also. The order of this difference is proportional to vc, as in the Doppler effect.

III. THE RCS OF A MOVING CONDUCTOR

When a conductor (or dielectric) moves with respect to a source, then, according to

Section II, the electromagnetic fields change in amplitude, direction, and frequency. These

3

changes are very small compared to the original fields, so we can expand the scattered fields

in terms of vc. According to standard text books [16], the scattered field can represented by

H′s(r) =

1

4π

∫

s′[(n′ ×H

′T )×∇′, ψ0]ds′, (5)

E′s(r) =

1

4π

∫

s′[ıω′µ′(n′ ×H

′T )ψ0 + (n′ · E′T )×∇′ψ0]ds

′, (6)

where ψ0 is the free space Green’s function [18]:

ψ0 =eık|r−r

′|

|r− r′|.

H′T and E

′T are the total electromagnetic fields at the surface of the scatterer,

H′T = H

′i +H′s, (7)

E′T = E

′i + E′s. (8)

The H0i is the radiating field in frame K and H′i (H

′s) is the incoming (scattered) field in

frame K ′. The relation between H′0 and Hi is

H′i = H0i − δH′, (9)

δH′ = β × Ei.

Inserting these equations into (6), we obtain

H′s(r) =

1

4π

∫

s′[(n′ × (H

′0T + δH′))×∇′ψ0]ds′. (10)

H′s(r) = H0s(r)− δH

′s,

δH′s =

1

4π

∫

s′[(n′ × δH′)×∇′ψ0]ds

′. (11)

Here, H0s(r) is the scattering field when the relative velocity of K and K ′ is zero. It must

be mentioned here that I ignore the relativistic effects that change the normal vector, the

free space Green’s function, and ds′, and assume that all these quantities are the same as

when K ′ is at rest with respect to K. Using Lorenz transformation by inserting v→ -v, we

obtain fields in K frame. Keeping only terms of order vcthen the scattered field in frame K

is:

Hs = H0s + δHs, (12)

δH = δH′s − β × Es.

4

The radar cross section (RCS) is [18]

σ = 4πr2Hs∗.Hs

Hi∗.Hi. (13)

By expanding the denominator,

σ = 4πr2(Hs∗.Hs)(

∞∑

l=0

l∑

m=−l

4π

2l + 1

|δH i|l

|H0i|l+1Y ′∗l,m(θ

′, φ′)Yl,m(θ, φ))2. (14)

The velocity of the moving object is very small compared to the velocity of light, therefore

we can write

δ Hi ≪ Hi.

In this situation, we may keep only the first and second terms in expansion.

σ ≃ 4πr2(Hs∗.Hs)2(4π

H iY ′∗0,0(θ

′, φ′)Y0,0(θ, φ) +4π

3

δH i

H0i2

1∑

m=−1

[Y ′∗1,m(θ

′, φ′)Y1,m(θ, φ)])2. (15)

σ ≃ σ0 +8π

3σ0 δH

i

H0i

1∑

m=−1

Y ′∗1,m(θ

′, φ′)Y1,m(θ, φ) + 2H0s.δHs

H0i2, (16)

where in above equation we have

Hs = H0s + δ Hs, (17)

σ0 = 4πr2H0s∗.H0s

H0i∗.H0i.

In equations (15, 16) θ′, φ′ (θ, φ) are the angles of δ Hi ( Hi) in spherical polar coordinates.

It is evident that θ′, φ′ contains the information about the direction of motion. The third

term in (16) can written as

2H0s2

H0i2

δHs

H0scos(γ) (18)

= 2σ0 δHs

H0scos(γ).

The order of this correction is proportional to vc. It must be noted that the boundary

condition for the total fields HT and Hs must be satisfied [3, 20]. Collecting results, we can

write following result for RCS up to order vcas:

σ = σ0

(1 +

8π

3

δH i

H0i

1∑

m=−1

Y ′∗1,m(θ

′, φ′)Y1,m(θ, φ) + 2δHs

H0scos(γ)

). (19)

5

FIG. 1. Geometry of incident plane wave and sphere.

Equation (19) has convenience form for relativistic corrections, because all quantities are

represented as static (v = 0) fields. As an example I assume a sphere with radius R and

moving with velocity v = vx. For simplicity I set (as shown in figure (1)):

Ei = E0xe−ıkr cos(θ). (20)

In this case we have:

β ×Ei = 0. (21)

By above simplification; the only corrections in RCS are due to last term in equation (12)

i.e. −β ×Es. According to standard text books, [16, 18, 19], the scattered fields at far field

limit for sphere can be written as:

5 10 15 20KR

0.0005

0.0010

0.0015

RCS Correction @m2D

FIG. 2. Relativistic corrections for mono static RCS as a function of kR (R is sphere radius) for

β = 10−5. This figure can be compared with figure (4) that indicate the mono-static RCS of same

sphere. The ratio of relativistic corrections and static RCS is (approximately) of order 10−4.

6

Esr∼= 0, (22)

Esθ∼= ıE0

exp(−ıkr)

krf(θ, φ), (23)

Esφ∼= ıE0

exp(−ıkr)

krg(θ, φ). (24)

With definitions:

f(θ, φ) = cos(φ)

∞∑

n=1

ınbn sin(θ)

∂P 1n(cos(θ))

∂ cos(θ)− cn

P 1n(cos(θ))

sin(θ)

, (25)

g(θ, φ) = sin(φ)

∞∑

n=1

ınbnP 1n(cos(θ))

sin(θ)− cn sin(θ)

∂P 1n(cos(θ))

∂ cos(θ)

,

an = i−n (2n+ 1)

n(n+ 1),

bn = −an((1 + n)jn(kR)− kRj1+n(kR))

((1 + n)h(2)n (kR)− kRh

(2)1+n(kR))

,

cb = −anjn(kR)

h(2)n (kR)

.

the results are shown in figures (1,2) for sphere moving with v = 103m/s. In figure (2)

the results are shown for relativistic corrections for mono-static RCS and we can compare

it with figure (4) that shows mono- static RCS for same geometry. The order of correc-

tions is (−30dBm←→ −40dBm). Comparing this results with mono-static RCS of sphere

(1dBm ←→ 10dBm) shows that the relativistic corrections and static RCS ratio approxi-

mately of order (-40 dBm). This result is predictable, because we expect that the relativistic

FIG. 3. The RCS relativistic corrections as a function of θ, φ and kR = 1 , β = 10−5.

corrections in first order are of order β = vc.

7

5 10 15 20K R

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Σmonostatic

Π R2

FIG. 4. Mono- static RCS for sphere as a function of kR .

IV. THE EFFECT OF TARGET ACCELERATION ON THE RCS

In the previous section, I only studied the case of constant velocity (the special theory of

relativity) and its effects on the scattering of the electromagnetic waves. In this section, I

will discuss the case of acceleration of the target and the possible effects of this acceleration.

All the relations are based on Newtonian mechanics and I don’t discuss accelerated frames

in relativity. The effect of acceleration can be taken into account by assuming inertial forces

on the conduction electrons [17]. This effective force on an electron is −mev and this is

equivalent to an electric field −mev

e[17].

E′ = E+mev

e. (26)

If we assume that v = u +−→Ω × r, where u is the linear velocity and

−→Ω is the angular

velocity, then we have (µr = 1)

∇× E′ = −µ0∂H′

∂t. (27)

In (27), H′ is [17]

H′ = H− 2me

−→Ω

eµ0. (28)

It can be shown [17] that solving the variable magnetic field near a non-uniformly rotating

body is equivalent to solving the problem of a body at rest in a uniform external magnetic

field:

Hequ = 2me

−→Ω

eµ0. (29)

According to the above equations, when a conducting sphere with µr1 lies in a uniform

periodic external field, its magnetic moment is V αH; where V is the volume of the sphere,

8

a is the radius of the sphere, and [17]

α = −3

2[1−

3

a2κ2+

3

aκcot(aκ)] (30)

κ =1 + ı

δ.

Here, δ is the penetration depth. In the case of a non-uniformly rotating sphere, we have

(δ ≪ a)[17]:

m =mea

5ς

15eµ0

d−→Ω

dt. (31)

In (31), ς is the conductivity of the sphere. If the incident wavelength is very large compared

to the radius of the sphere (Rayleigh scattering), then we can use the following relations

between the incident and the scattered fields [16]:

Einc = d0E0 exp(ıkn0.r), (32)

Esc =k2 exp(ıkr)

4πε0r[(n× p)× n]−

µ0 exp(ıkr)

4πrc[φ sin(θ)

∂2m(t)

∂t2].

In the above equations, d0, n0 (d, n) are the incident (scattered) electric field polarization

and plane wave propagation direction, respectively (p is the electric dipole moment of the

sphere and I assumed that m(t) = m(t)k.). The first term represents the electric dipole scat-

tering and the second term is the electric field radiating from the time dependent magnetic

moment. In this case, the differential scattering cross section ( σ4π) becomes

dσ

dΩ= |

k2

4πε0E0

(p.d∗)d+µ0

4πE0cφ sin(θ)

∂2m(t)

∂t2|2. (33)

For the special definition of d0, n0 ,d, n, as shown in Figure (5), we have

dσ‖dΩ

=k4a6

2cos2(θ) +

1

2(µ0m(t) sin(θ)

4πcE0

)2 (34)

dσ⊥dΩ

=k4a6

2+

1

2(µ0m(t) sin(θ)

4πcE0)2.

It is evident that the resulting scattering cross section is completely different from the non-

rotating case. An unusual result appears in (34), and the scattering cross section depends

on the incident electric field strength. This is an indication of the presence of nonlinear

effects in the scattering process.

9

FIG. 5. The geometry of the incident and scattered fields.

V. RESULTS AND DISCUSSION

In the present paper, I investigated the possible effects of target movement on the scat-

tering cross section, i.e., the RCS. The theoretical calculations show that when the target

has a velocity with respect to the source, then the incident plane wave parameters change.

These changes include the frequency (the Doppler effect), the incident angles, and the field

vectors (E , H) (in magnitude and direction). I only studied the changes in the fields

and the results showed that a moving target RCS is different from the motionless (stand-

ing or static) case. This difference arises from changes in the electric and magnetic fields

due to relativistic effects (constant velocity case) and non-relativistic effects (an accelerated

object). When the object moves with constant velocity then, according to the special the-

ory of relativity, the observer that moves with the object measures different field vectors

and frequencies. The changes in the frequency can explained by the phase invariance of

the energy-momentum four-vector (hν/c,p) under Lorentz transformations for the incident

photons. The field changes can be obtained from the field strength tensor transformation.

As with the Doppler effect, the order magnitude of the changes in the field is proportional

to vc. This means that the moving corrections are very small compared to the static case, as

with the Doppler effect. When the incident frequency is on the order of MHz–GHz, we can

measure the Doppler frequency, but in the case of fields, the strength of the field is not large

enough, and so the detection of relativistic effects becomes difficult. For accelerated frames,

I used the results of Newtonian mechanics and added an inertia force to the conducting

electrons’ equation of motion. This is equivalent to changes in the effective electric field

10

(or current density). When we use the effective electric field or effective current density,

it can be shown [17] that the magnetic moment of the accelerated object depends on the

angular acceleration (not the total acceleration). This means that the magnetic moment of

a non-uniformly rotating sphere is different from that of a non-rotating one. Changes in

the magnetic moment directly change the RCS, and the differential scattering cross section

was calculated in the Rayleigh scattering regime. In this paper, I used the far-field limit

of the radiating electromagnetic fields with a time-dependent magnetic moment, and didn’t

solve Maxwell’s equations. For a non-uniformly rotating sphere, the solution outside of the

sphere is similar to that for a non-rotating one, i.e., H′ = 0. On the surface of the sphere,

H′ is continuous, and outside the sphere, at infinity, H′ becomes −Hequ. The effect of a

non-uniform rotation in a ring appears as an e.m.f and is known as the Stewart–Tolman

effect. The other possible effects of this movement, such as small surface deformations and

thermal effects, were not included in this paper. These effects generally depend on the shape

and structure of the moving object and can be investigated as independent parameters in

RCS calculation.

VI. ACKNOWLEDGMENTS

I would like to thank the Research Council of the University of Tehran and the Institute

for Research and Planning in Higher Education for financial support and grants provided

under contract No. 138-569.

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